Sorry, Extra income tax withholding will only be added in box 2 (box 17 for state). Box 12 is for other things like contributions to a 401K plan, health insurance see What are the Codes for W2 box 12?
https://ttlc.intuit.com/turbotax-support/en-us/help-article/taxation/letter-codes-box-12-w-2-mean/L6...
